Stiffness Control of Variable Stiffness Link Using a Conductive Fabric Based Proximity Sensor

With an ongoing interest to create solutions allowing for safe physical interaction between humans and robots, new approaches to the design of robot arms are appearing. One example is the recently developed Variable Stiffness Link (VSL) -a new type of inflatable soft robot link made of flexible, yet inextensible fabric sleeves and capable of changing its stiffness. At high stiffness, the VSL can perform comparable to a rigid link within a standard robot arm, e.g., carry out picking and placing tasks in a factory environment. However, by decreasing the air pressure in such links their stiffness also decreases, and the robot arm's compliance increases. Hence, when a human erroneously intrudes into the robot working area, safety can be ensured by simply reducing the pressure in the robot's links. In this context, a major challenge is to determine whether a human is within the robot's range. In this study, we propose to integrate a proximity, capacitance-based sensor made of conductive fabric with a VSL. Since the proposed proximity sensor is made of fabric, it is ideally suited for integration with the VSL: the sensor material is flexible and at the same time inextensible and as such can be used as the outer layer of a VSL without affecting its stiffness controllability. An appropriate control strategy was developed capable of changing the stiffness of the VSL depending on the distance between the VSL and a human or object nearby using the new sensor. The proposed sensor/control system determines the distance to a human or object in the vicinity of the VSL by evaluating the capacitance as measured by the integrated proximity sensor and adjusts the link's stiffness accordingly to ensure safety. It was experimentally validated that, our new approach was capable of reducing the VSL's stiffness when a human was approaching, even before contact occurred.